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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
83
DOI: 10.4408/IJEGE.2011-03.B-010
FORMATION MECHANISMS OF DEBRIS-FLOW MATERIAL
ON SLOPES INROUDBAR-E-GHASRAN, IRAN;
A CASE STUDY OF THE WEATHERING IMPACTS
m
oJGan
H
adi
MOSLEH
(*)
& f
aRzan
RAFIA
(**)
(*)
Geoenvironmental Research Centre, Cardiff University, Cardiff, UK Former Postgraduate Student,
Bu-Ali Sina University, Hamedan, Iran
(**)
Kavoshgaran Consulting Engineers Company, Tehran, Iran
INTRODUCTION
Physical and chemical properties of soft sedimen-
tary rocks can be highly affected by weathering process
resulting in the alteration of the engineering behaviour
of debris-flow materials, deposited on the slopes. Char-
acterisation of the weathering process of parent rocks
in debris-flow material is a crucial issue in assessing
the stability of slopes that are covered with debris-flow
materials (y
akota
& i
wamatsu
, 1999). The weathering
process generally is defined as the breakdown of solid
bedrock, through different mechanisms such as chemi-
cal, biological, and mechanical process, as well as com-
plete transition between bedrock and soil. Among all
weathering mechanisms chemical and mechanical ef-
fects are the most dominant factors in breaking down
the rocks into fragments (t
imotHy
, 2008).
This paper aims to investigate the important weath-
ering indices including petrographical, chemical and
engineering indices and a discussion of their limitations
and usefulness with regard to climate condition. Some
of the existing methods for determination of weathering
indices have been tested in experiments on two common
rocks of the Eocene Karaj formation: gray-green vitric
tuff and dark gray tuffaceous limestone. The results are
discussed and the most dominant type of weathering in
the region is discussed in the conclusion section.
LOCATION AND GEOLOGY
The investigated area in this paper is located in
the northeastern part of Tehran province in Iran, along
ABSTRACT
Debris flows are widespread in many steep slopes
in northeastern of Tehran province, Iran. The slopes
in that region are mostly covered by a relatively thick
mantle of weathered debris. Parent rocks mainly con-
sist of vitric tuff and tuffaceous limestone of middle
Eocene age. Several factors such as geomorphology,
structural geology, tectonics and weathering as well
as construction works trigger instability of slop in
that area. Among various factors on the formation of
debris-flow material, tectonics and weathering have
been found to be the most influential factors.
This study is focused on the formation mecha-
nisms of debris-flow material and weathering indices
of the parent rocks. The effects of weathering on some
slopes in the first 15 km of the main road from Zard-
band to Meygoon were studied. Soil and rock samples
were collected from the currently stable slopes as well
as material from an old landslide. Geotechnical prop-
erties of the debris mantle were investigated using a
series of field and laboratory tests. In addition, differ-
ent weathering indices comprising of petrographical,
engineering and chemical tests have been conducted
in order to assess the most dominant type of weather-
ing. Finally, it was concluded that mechanical weath-
ering is the most influential process in the formation
of talus material in the study area.
K
ey
words
: debris flow, weathering, weathering index, tuff
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M.H. MOSLEH & F. RAFIA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
or close to the urban development activities whilst old
landslides are located almost far from the main roads.
From geological point of view, the widest tectonos-
tratigraphic units are Eocene pyroclastic and volcanoclas-
tic sequences (Karaj formation) covering a wide region
on the southern slopes of the Alborz Range (Figure 3).
The Eocene Karaj Formation is exposed in most
parts of the south central Alborz Range, bounded by
major thrust faults. This formation is a volcanic-sedi-
mentary unit that consists of a variety of tuffs. Based on
petrographical studies the tuffs in this region are mainly
classified into three main categories including crystal-
line, vitric, and lithic tuffs (NGDIR, 2006). The lithic
tuffs contain more sedimentary components and mainly
include shale and siltstone- sandstone tuffs with gray to
dark gray color (y
assaGHi
et alii, 2005). Massive parts of
volcanic tuffs in the study area mainly consists of green-
gray vitric tuff and dark gray tuffaceous limestone.
Slopes in the region are mostly covered by rela-
tively thick fragmented materials and talus. The origin
of these materials is mainly Karaj formation. Tectonics
and weathering are known as two major factors respon-
sible for formation of these materials in the region. The
effects of tectonics and fault activities in the formation
of talus in the study area have been studied and reported
in the literature (e.g., t
CHalenko
et alii, 2007; N
asiRi
,
2004; a
mbRaseys
& m
elville
, 1982). In this paper, the
effects of weathering on formation of talus material are
investigated and the most influential types of weather-
ing in the region also are discussed.
15 km of the main road which connects Zardband vil-
lage to Fasham city (Fig. 1).
The region is located in the high mountainous area
of Roudbare- Ghasran with altitude ranging between
2500 and 4000 m. Based on the recorded data in the
l
andslide
d
atabase
of
i
Ran
(2007), several old land-
slides and debris-flows have occurred during the last
hundred years in this region. In the last ten years, the
number of reported landslides has increased consider-
ably, reported to be related mainly to the urban develop-
ment and road widening (l
andslide
d
atabase
of
i
Ran
,
2007). Figure 2 shows some of the existing landslides
in the study area. Accordingly, the location of recent
landslides is mostly in the vicinity of the main roads
Fig. 1 - Location of the investigated area
Fig. 2 - Location of the existing landslides in Roudbar-
e-Ghasran region (Mountains view from Google
earth, 2010)
Fig. 3 - Geological map and location of the study area
(NGDIR, 2006)
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FORMATION MECHANISMS OF DEBRIS-FLOW MATERIAL ON SLOPES INROUDBAR-E-GHASRAN, IRAN;
A CASE STUDY OF THE WEATHERING IMPACTS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
85
purposes; tests such as point load test, and the Schmidt
rebound hammer test were carried out as well as labora-
tory tests including physical properties, slake durability,
wetting-drying, freezing-thawing and soundness tests.
PHYSICAL PROPERTIES OF PARENT ROCkS
(INTACT ROCk)
Physical indices such as specific gravity, dry and
saturated densities, porosity and void ratio were deter-
mined for the rock types following the standard test
procedures suggested by ISRM (1981). Schmidt Re-
bound hammer test also was carried out according to
standards in ISRM (1993). The uniaxial compressive
strength for the two rock types was determined based
on Point load test on 20 irregular lumps and 10 cylin-
der shape samples. Physical properties of the studied
parent rocks are presented in Table 1.
The point load test index shows that compressive
strength of tuffaceous limestone is two times larger
than vitric tuff. Accordingly, tuffaceous limestone
and vitric tuff were classified as class A (Very high
strength) and class B (High strength) rocks, respec-
tively (d
eeRe
& m
illeR
, 1966).
SLAkE DURABILITY TEST
The slake durability test is regarded as an initial
assessment of the weatherability of rocks. This test
is intended to assess the resistance of rock samples
to fragmentation (f
Ranklin
& C
HandRa
, 1972).
Rock samples were put into an apparatus that com-
prises two sets of drums and were rotated in water
that had a level of about 20 mm below the drum
axis. After slaking for the period of 10 minutes,
samples were dried in an oven at a temperature of
105°C for up to 6 hours. The test was conducted
over two cycles, the weight loss of particles of 10
CLIMATE CONDITION
According to the Köppen climate classification
(P
eel
et alii 2007), northeastern Tehran has a cold
semi-arid climate whilst the weather is often cooler
in the hilly areas of the north as compared to the flat
southern part of Tehran. Based on the collected data
by the Iran Meteorological Organization, the average
annual precipitation is between 500 to 570 mm and
the average annual temperature is about 12°C. The
hottest month in the region is July with a mean mini-
mum temperature of 22°C and a mean maximum tem-
perature of 34°C. The coldest month is January with
a mean minimum temperature of -1°C and a mean
maximum temperature of 7°C (IRIMO, 2006). Figure
4 shows the meteorological records of the north Te-
hran station between 1990 and 2005.
FIELD AND LABORATORY INVESTI-
GATIONS
In order to investigate the factors that affect the
weathering of parent rocks, samples were collected
from different locations where slopes are covered by
relatively thick layers of talus materials. Two types of
tuffs were found dominantly as the parent rocks of the
debris materials on the selected slopes including vitric
tuff and tuffaceous limestone. Existing data from oc-
curred landslides in the study area suggest that the ori-
gin of debris material is Karaj formation.
Figure 5 shows one of the sampling points near
Aminabad village. Field and laboratory experimental
studies were carried out on collected samples of the se-
lected slopes. The selection of experimental studies was
mainly based on the determination of weathering indi-
ces. Microscopic thin-sections were prepared for petro-
logical studies; the X-Ray Fluorescence (XRF) test was
performed on rock samples for chemical characteristics
Fig. 4 - Average precipitation between 1990 and 2005 and
the number of days with a minimum temperature
equal to 0°C and below
Fig. 5 - Debris material on a steep slope near to Amina-
bad village
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M.H. MOSLEH & F. RAFIA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Therefore, the Soundness test was carried out using
saturated solution of sodium sulphate according to
ASTM STP 169C (f
oRsteR
, 1994). The results of
soundness test are presented in Table 1.
RESULT AND DISCUSSION
The climatic conditions have an important effect
on weathering processes of rock materials, in addi-
tion to the other environmental factors such as hydro-
sphere and topographical conditions. The relationship
between mean annual temperature and mean annual
rainfall can be used to describe the regional impor-
tance of mechanical weathering (P
eltieR
, 1950).
Figure 4 shows the number of wetting-drying and
freezing-thawing cycles which describes the impor-
tance of climatic condition in a period of 15 years.
A close relationship between climate conditions and
weathering rate of rocks has been well established
(e.g., w
eineRt
, 1964; s
andeRs
& f
ookes
, 1970).
Every year, the study region is covered by snow
for a period of 4 to 5 months. While rainfall is domi-
nant during spring, hot and dry weather conditions are
generally observed during the summer. Therefore, when
seasons are considered altogether, it is apparent that
physical weathering processes (i.e., freezing-thawing)
may control the fragmentation of bedrocks. Changing
the water phase might cause a 9% volumetric expan-
sion resulting in 200 MPa pressure which is caused by
freezing process (o
llieR
, 1984). The stress may exceed
the tensile strength of rocks several times within a year.
rock lumps retained in these wet-dry cycling tests
were determined (Fig. 6).
The slake durability index (Id
2
) of vitric tuff and
tuffaceous limestone were 99.86% and 99.92%, respec-
tively. Both rock types seem to have high resistance in
the slake durability test as the index of durability tends to
more than 90%. Rocks are ranked as class 6 or extreme-
ly high strength according to s
adisun
et alii (2002).
WETTING- DRYING AND FREEZING AND
THAWING TESTS Wetting-drying and particularly
freezing-thawing processes are significant atmos-
pheric events causing physical disintegration of rock
materials in a long time. f
ookes
et alii (1988) men-
tioned that crystallization processes such as freezing-
thawing can be deleterious to rock materials over
short periods of time. A 9% volumetric expansion
upon phase change of water is known as the major
reason of rock breakdown (o
llieR
, 1984).
The effect of these processes was quantified by
performing wetting-drying and freezing-thawing tests
on two major rock types of the region according to the
s
adisun
et alii (2002) and ASTM D5312. The results
are represented in Table 1.
SOUNDNESS
In the Soundness test, samples are exposed to
cycles of wettingdrying by a sulphate solution. After
each drying cycle, the dehydrated sodium or mag-
nesium sulphate precipitated in pore spaces of ag-
gregates causes expansion during the soaking cycle.
This expansion simulates the expansion of water
upon freezing (f
oRsteR
, 1994).
In order to determine the type of sulphate solu-
tion (sodium or magnesium sulphate), natural water
in the study area was collected and analysed in Ana-
lytical Chemistry laboratory at Bu-Ali Sina Univer-
sity. The amount of sodium and magnesium in natu-
ral water was 44.90 ppm and 1.99 ppm, respectively.
Fig. 6 - Slake durability test and samples condition after
second cycle. a) Vitric tuff, b) Tuffaceous limestone
Tab. 1 - Physical and engineering properties of parent rocks
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FORMATION MECHANISMS OF DEBRIS-FLOW MATERIAL ON SLOPES INROUDBAR-E-GHASRAN, IRAN;
A CASE STUDY OF THE WEATHERING IMPACTS
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
87
Considering only the temperature variations around 0°C
in Figure 4, there were at least 15 freezing-thawing cy-
cles between 1990 to 2005. Although, within each year,
there were a number of times the temperature variations
around 0°C produced freezing- thawing cycles (IRIMO,
2006). However, the availability of water for this physi-
cal process is very important. Therefore, in determina-
tion of the physical processes, the precipitation needs to
be used as an input parameter in freezing-thawing cycle
determination as well as temperature variations.
In order to quantify the effect of climate condi-
tion on green-gray tuffs and dark gray tuffaceous
limestones, weathering indices were investigated.
Weathering indices are widely used in studies of both
modern and ancient in situ weathering profiles, dem-
onstrating the impact of climate on bedrock weath-
ering (e.g., n
eall
, 1977). The most commonly used
methods for weathering indices can be categorized as
mineralogical-petrographical, engineering, and chem-
ical indices (G
uPta
& s
esHaGiRi
, 2001). The common
weathering indices are discussed below with regard to
their applicability for the assessment of the weather-
ing effects on different bedrock in the study area.
MINERALOGICAL/PETROGRAPHICAL INDICES
The number of micro-fractures is considered
as petrographical index (d
ixon
, 1969) by making a
squared traverse using the point counting method. In
order to investigate the effect of petrographical in-
dex on weathering of bedrocks, 13 thin sections were
prepared and studied with a polarizing petrographic
microscope. Figure 7 shows one of the microscopic
images. As shown in Figure 7, microstructures are not
developed in the fabric of both rock types, therefore
petrographycal index can not be used as a factor of
weathering evaluation in this study area.
ENGINEERING INDICES
The physical characterisation of the bedrock was
determined based on several index properties such
as dry specific weight, porosity, saturated moisture
content, and compressive strength. All samples used
in physical characterization tests were prepared ac-
cording to the ISRM (2007) suggested methods (Ta-
ble 1). Figure 8 indicates the histogram of engineer-
ing indices as comparative values. Both rock types
have similar static and dynamic durability indices,
but there is a considerable difference between Freez-
ingthawing index and Soundness index of both rocks
(e.g., vitric tuff and tuffaceous limestone).
CHEMICAL INDICES
Chemical weathering indices, sometimes referred
to as indices of alteration, are commonly used for
characterizing weathering profiles. Chemical weath-
ering indices incorporate bulk major element oxide
chemistry into a single value for each sample.
Chemical indices studied for the weathering of
tuffaceous limestone and vitric tuff are represented in
Table 2. According to the results represented in Table
2, Alkaline Ratio and Chemical Index of Alteration
show that vitric tuffs have more weathering potential
than tuffaceous limestone. Product index and Silica-
Titanium Index are similar in both rock types. Finally,
only SAR represents higher rate of weathering in tuf-
faceous limestone than vitric tuff.
Although the indices based on chemical and pet-
rographical characteristics can explain the variations
in the basic characteristics of rock material caused by
weathering (such as chemical alteration, fabric and
mineralogical changes), their evaluation is experi-
mentally difficult. However, a weathering index based
on engineering properties is preferred over other kind
Fig. 7 - Microscopic thin section under XPL and PPL (magni-
fication: 250×.) a) tuffaceous limestone, b) vitric tuff
Fig. 8 - Engineering indices and weathering ratio of par-
ent rocks
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M.H. MOSLEH & F. RAFIA
88
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
is the dominant mode of weathering in the region.
Because, the Soundness test simulates the expan-
sion of water during a freezing process, the result
of this test also supports the dominance of physical
weathering (the weight loss of green-gray tuffs is
significant).
In conclusion, the results of this investigation indi-
cates that mechanical weathering is the most influ-
ential process in breakdown of parent rocks of Karaj
green-gray tuffs and formation of talus material on the
slopes in northeastern Tehran province. In addition,
based on the results of freezing-thawing and Sound-
ness tests, about four percents of vitric tuffs break
down into the small fragments in every five freezing-
thawing cycles. Hence, talus production rate in the
study area is estimated to be considerable and effec-
tive on the formation and instability potential.
ACKNOWLEDGEMENTS
This research was part of MSc thesis of first author at
Bu-Ali Sina University. The first author would like
to acknowledge the support of all academic staffs
and the supervisory team as well as Kavoshgaran
Consulting Engineers.
of indices, considering that the criteria used can be
easily and quickly determined in the field as well as
in the laboratory. The ability to predict the engineer-
ing performance of weathered rock is also important
for geotechnical purposes (G
uPta
& s
esHaGiRi
, 2001).
CONCLUSION
The most dominant type of weathering in the re-
gion is defined according to climatic condition and
an N-value (w
eineRt
, 1980). The N-value provides
an indication of the dominant mode of weathering of
rock material. The N-value is based on the following
formula:
N = 12E
j
/ P
a
where N is climatic index or N-value, E
j
is evapora-
tion amount during the warmest year in the region, and
P
a
is the total annual precipitation both in mm. In arid
areas, the N-value is more than 5 and mechanical frag-
mentation of rock is predominant. In humid areas, the
N-value is less than 5 and chemical decomposition of
rock occurs (w
eineRt
, 1980). From the meteorological
records of the study area, E
j
and P
a
are 2374 mm and
421 mm, respectively. Therefore, the N-value for this
region is 5.64 indicating that mechanical disintegration
Tab. 2 - Chemical indices of parent rocks
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Statistics